A calcined coke produced in a delayed coking plant is used for an aggregate of an artificial graphite electrode for electric steel manufacturing, an aggregate of an electrode for aluminum refining and an aggregate for a special carbon material, and in addition thereto, it is used for a recarburizer and fuel. On the other hand, cracked distillate is used for gasoline, a base raw material for chemical products, fuel and black oil.
In general, artificial graphite for an artificial graphite electrode for electric steel manufacturing and a special carbon material is produced by crushing a calcined coke produced in a delayed coking plant, adjusting the crushed to a certain particle size distribution and baking it after blending with binder pitch and molding, and then impregnating the baked one with impregnating pitch, secondarily baking, repeating this several times if necessary, followed by subjecting to graphitizing treatment. The graphitizing treatment is a step in which thermal treatment is carried out at about 2500 to 3000° C.
Isotropic coke (amorphous coke) is desired particularly as a coke for a special carbon material among cokes for artificial graphite. This is because preventing anisotropy from appearing when carrying out cold isostatic press (CIP), mold pressing and extrusion pressing. Accordingly, a coke having a large thermal expansion coefficient is desired since if a coke has a small thermal expansion coefficient, the orientation is produced by pressure. Further, a carbon material is excellent in a heat resistance and therefore is used on a severe condition, for example, on a high temperature condition, so that a material having a high density and high strength is usually desired.
In general, a thermal expansion coefficient of artificial graphite is determined by a coke which is an aggregate. It is said that a thermal expansion coefficient of a coke is closely related to growth of a crystal structure of the coke. When a crystal structure of a coke grows to a large extent, a thermal expansion coefficient of the coke becomes smaller. Further, a density and strength of artificial graphite are determined as well by a coke which is an aggregate. A graphite structure is formed in a coke by graphitizing treatment, and irreversible shrinkage and expansion are brought about during the course thereof. A bulk density of artificial graphite and strength thereof closely related to the bulk density are determined by the degree of this shrinkage and expansion.
Known as a method of controlling a thermal expansion coefficient of a coke for a special carbon material is, as described in Japanese Patent Publication No. 60-3118/1985, a method of producing an amorphous coke by adding to coal tar pitch, a quinoline-insoluble accumulated substance by-produced when producing a needle coke from a coal tar base raw material. However, an ash in coal tar is contained in a quinoline-insoluble substance, and it is not preferred for a high purity carbon material prone to be damaged by metallic impurities.
Further, a method of producing an amorphous coke by adding carbonaceous fine powder such as carbon black to coal tar pitch is described in Japanese Patent Application Laid-Open No. 2-69308/1990. However, this method has the problem of a dispersibility of the carbonaceous fine powder in a coking step and the problem in that the shrinkage factor after graphitization is low. This requires to add a large amount of carbon black in a special carbon material whose product requires a high density and high strength and provides the problem in that the dispersibility is further deteriorated.
A treating method in which waste plastics (high molecular materials) are thermally decomposed together with heavy hydrocarbon oil by a coker is described in Japanese Patent Application Laid-Open No. 6-80970/1994 as an inexpensive treating method therefor, but it is not described therein to control a thermal expansion coefficient of an amorphous coke for a special carbon material.
Accordingly, it has been an important object to develop an amorphous coke as a raw material for a special carbon material of a high grade which can readily be controlled in a thermal expansion coefficient and does not contain metallic impurities and which shows a high shrinkage factor in graphitization.
In light of such existing situation, an object of the present invention is to provide an amorphous coke which can readily be controlled in a thermal expansion coefficient and whose molded article shows a high shrinkage factor in graphitization, thereby allowing the resulting graphitized product to have a high density and high strength and a production process for the same.